1. Technical Field
This invention relates generally to the field of medical video equipment. More specifically, the invention comprises a synchronized light source for rolling shutter imagers, i.e. a control system for an LED light source for use with an endoscopic or similar camera system with a CMOS-type imager.
2. Description of the Related Art
There are many types of light sources and related control systems for endoscopic video. Many of these use a high-output lamp or incandescent bulb such as metal halide, quartz-halogen, or xenon types. These lamps are typically used in a mode of a fixed luminous intensity, and the intensity of light transmitted out of the device is controlled by means of a moving and variable mechanical aperture, an example being an iris, which blocks some or all of the light being generated by the lamp to the receiving fiber optic light guide.
Control of the intensity of this light is important for various functional and safety reasons. Improper light levels can cause under-exposure or over-exposure, forcing the camera system to overcompensate in ways that reduce or limit the image quality and camera performance. Safety concerns involving high light transmission can include skin and tissue burns, and possible ignition of flammable materials.
Adoption of high power Light-Emitting Diode (LED) technologies is becoming common in many fields and industries, including medical endoscopy, reducing overall power and cost while increasing product reliability and service life. LED light output intensity can be controlled by varying the amount of electrical current driving the LED device. This method has several drawbacks including inefficiency, a practical minimum for the lower end of light output, and a tendency for the color, or output wavelength(s), of the LED lamp to drift with intensity. Being a solid state device, it is also common practice to control overall LED light output in a switched Pulse-Width Modulation (PWM) fashion. For the human eye, film cameras, and some video cameras, this pulsed light is effectively integrated into an “average” that when applied at an appropriate frequency can be virtually indistinguishable from a constant light source. For human vision persistence, this frequency is typically about 30 pulses per second.
The PWM method of light intensity control works well with video cameras with frame-transfer imagers such as CCDs (Charge-Coupled Devices), so long as the switching frequency of the light is equal or greater than the camera's rate of frame capture, typically 60 exposures per second for a video camera. It is convenient to use an integer multiple of the frame rate, such as double, for the PWM switching frequency. It is also beneficial to synchronize the light source with the camera's frame rate to avoid a frequency mismatch with can result in a beating, flickering or “strobing” image.
However, with the recent adoption of CMOS (Complementary Metal Oxide Semiconductor) imagers into medical endoscopy, PWM-controlled LED light sources present a challenge. Specifically, the challenge relates to CMOS imagers with a “rolling-shutter”-type exposure architecture, the most common type, as these are not frame-transfer devices. Instead, each line of the raster image is exposed in a cascading overlapped sequence, with lines being read out while other lines are exposing. The exposure of one line will thusly never start and stop at the same time as another line, even though the resultant time duration is same, and their exposures will overlap one another in time.
Thus, traditional frame-rate based PWM control is unsuitable, as individual lines or groups of lines may have a significantly different light exposure than other lines, creating undesirable regions of differing exposure within the image. The number of different regions of exposure is equal to twice the relative PWM frequency, and the complementary size of the light and dark regions being directly proportional to the PWM duty cycle. A PWM system not synchronous to the imager frame rate would additionally cause a “roll” of this effect, where the output video would have these regions in different places of the current image frame than the subsequent image frame.
Rolling shutter imagers can control exposure via an internal shutter mechanism. This is done by setting control registers inside the imager that specify the number of line times, as denoted by an HSYNC signal, that the imager lines are exposing. The granularity of this exposure control is thusly in one-line increments, and can vary from exposing for a full frame duration to exposing for only one line duration. For a full frame exposure time, this is typically the total number of lines in the imager minus one, as a line typically cannot be exposed while it is being read out, and if the imager is operating at 60 frames per second, the exposure time would be approximately 1/60 of a second, or 16.67 msec. On the other hand, if the imager were to have 1000 imager lines, then the shortest exposure duration achievable by the imager shutter when imaging at a rate of 60 frames-per-second is 1/60/1000 of second, or 16.67 usec. This is a particularly significant limitation, as exposures of 1/100,000 of a second (10 usec), or shorter are often necessary in medical imaging situations if there is no further control of the brightness of the light source.
What is desired, therefore, are methods for variable pulse width and PWM control of an LED light source for use with CMOS imagers capable of producing imager exposures equivalent to times shorter than the typical readout time of one imager line that do not produce an undesirable exposure effect to the video image.